This invention relates to a navigation system mounted on a vehicle.
Navigation systems have been proposed that compute an optimal route from a starting point to a destination and guide a vehicle along the route.
The vehicle may get off the optimal route through the driver's carelessness (hereinafter "off-route deviation"). Japanese Patent Publication No. 2-18488 and Japanese Laid-Open Patent Application No. 2-141899 disclose systems in which, when the vehicle gets off the original optimal route, a new optimal route is recomputed from the vehicle's present position to the destination. The vehicle is then guided along the new optimal route.
With such a navigation system, however, if the distance is long from the vehicle's present position to its destination, recomputing the new optimal route takes considerable time, so the navigational guidance information cannot be immediately presented to the driver. This delay causes the driver to feel uneasy. In the worst case, during the prolonged recomputation the vehicle goes straight through an intersection where it should have turned. As a result, the partial recomputation becomes useless, and the vehicle cannot be guided along the new optimal route.